In diesel internal combustion engines, the fuel injection profile may include multiple injection events within a single injection cycle, such as a at least one pilot injection followed by at least one main injection, in order to reduce ignition delay and exhaust emissions, as well as combustion noise. In addition, a common rail fuel system is may be used in diesel engines which may include a high pressure fuel pump that delivers fuel to a fuel rail associated with a group of cylinders. The fuel rail system maintains sufficient fuel pressure for injection while distributing fuel to the injectors, which all share fuel in the common rail. The rail volume acts as an accumulator in the fuel system and dampens pressure fluctuations from the pump and fuel injection cycles to maintain nearly constant pressure at the fuel injector nozzle. The accuracy of any given fuel injection event is dependent on the ability to maintain a sufficiently constant fuel rail pressure and thus, any deviation in the rail pressure from the intended value may result in inaccurate fuel delivery, which may, in turn, degrade the engine performance.
However, when the engine is operating at high engine speed and load conditions that demand higher fuel injection quantity, the fuel pump might not be able to supply enough fuel into the fuel rail to maintain the high fuel rail pressure. Thus, during high engine speed and load conditions, the engine may be derated when the high pressure fuel pump is unable to maintain sufficient rail pressure. Alternatively, fuel pump size may be increased in order to supply sufficient fuel during high fuel quantity demand. However, this leads to high parasitic loss in the pump. One example approach for preventing a deviation in rail pressure is shown by McCormick et al. in U.S. Patent Application Publication No. 2003/0089334. Therein, pilot injection timing is determined based on the fuel pressure in the common rail and the operating state of the engine. When the common fuel rail pressure is high, the pilot fuel injection timing is advanced in order to maintain the interval period which separates the pilot fuel injection and main fuel injection. However, the inventors herein have identified potential issues with such an approach. As one example, advancing pilot injection timing at high engine speed/load may still degrade engine performance if the fuel pump capacity is insufficient to supply enough fuel into the common rail and thus results in the inability to maintain high pressure in the fuel rail.
The inventors herein recognized that by reducing an actual total number of pilot and/or main fuel injections provided to a cylinder when a rail pressure deviation is detected (e.g. when the actual fuel rail pressure is less than a target fuel rail pressure), less fuel will be sent back to the fuel tank from the injectors after the fuel injections and more fuel may be conserved within the fuel rail to maintain the fuel rail pressure. Therefore, the issues described above may be at least partly addressed by a method comprising: reducing a total number of fuel injections provided to a cylinder in a given cylinder cycle responsive to a threshold reduction of pressure in a fuel rail. In this way, sufficient fuel may be provided to sustain high load/speed engine operation while maintaining a relatively constant high pressure in the fuel rail.
As one example, during high engine speed and load conditions, a target fuel rail pressure may be obtained (e.g. from an engine look-up tables), and if the actual fuel rail pressure is lower than the target fuel rail pressure, then the number of injections may be reduced to conserve fuel amount by preventing injector fuel return from the fuel injections. At the same time, depending on the engine load condition, the pilot and/or main injection timing and quantity may be adjusted in order to provide sufficient fuel into the combustion chamber to accommodate the change in the engine torque demand and to maintain a constant common fuel rail pressure.
In one example, during high engine load condition (e.g. when high torque demand is requested) and actual fuel rail pressure drops below a threshold pressure, the total number of pilot fuel injections may be reduced in order to prevent peak in-cylinder pressure exceeding a pressure limit. In addition, the remaining pilot injection pulsewidth may be decreased and its injection timing may be retarded in order to reduce combustion noise. At the same time, the main injection timing may be advanced relative to the engine compression stroke to reduce ignition delay. By shifting pilot injection timing closer to the main injection timing, cylinder pressure may be increased to promote fuel combustion. Further, the main injection pulsewidth may be increased in order to provide sufficient fuel supply to satisfy the increased in engine torque demand.
In another example, it may be possible for the actual fuel rail pressure to drop below a threshold pressure during low load conditions, such as if the fuel pump is degraded or if fuel temperature above a threshold (or if the fuel viscosity is below a threshold). In these conditions, it may be desirable to maintain the pilot injections to mitigate engine noise and instead one or more of the main injections may be reduced to decrease engine's power output. Further, pilot injection timing may be advanced and a smaller pulsewidth may be provided. The main injection timing may be retarded in order to increase exhaust gas temperature. A larger quantity of main fuel injection may be provided based on the desired total fuel amount. In this way, an accurate amount of fuel may be delivered into the engine while the fuel rail pressure is maintained at a relatively constant pressure.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.